Trauma, rheumatoid arthritis, and degeneration are the major problems in the upper cervical spine (occiput-C1-C2 joint complex). They result in pain, deformity and neurological dysfunction. An important and clinically recognized entity is spinal instability. It relates the major problems of the spine to the clinical results. Given an unsable spine, due to trauma, arrthritis, or deeneration etc., surgical reconstruction is used to stabilized the unsable spine. Several questions remain unanswered. What constitutes instability in the upper cervical spine? What are the different types of instabilities and their measurements? What are the mechanisms of injuries in this region? What roles doe each of the spinal ligaments play in providing stability? What type and magnitude of instability leads to vertebral artery insufficiency? What are the normal anatomic relationship between vertebrae, nerve roots, the spinal cord and vertebral artery? Given an unstable upper cervical spine, what are the relative advantages and disadvantages of different surgical procedures? It is hypothesized that an integrated set of studies, which applies modern methods of biomechanical investigation, is needed to address the above clinical and basic science questions. The proposal, which consists of five studies, provides a comprehensive approach to the study of the upper cervical spine. For this region, we propose to determine, using fresh human cadavers and cadaveric specimens, the following: (1) Anatomy. Detaialed and quantified three-dimensional anatomy, using computer graphics; computer graphics (2) Biomechanics. Normal three-dimensional movements andphysical properties; (3) Instability. Determination of Instability thresholds and the role od ligaments in stability; (4) Trauma. High speed trauma and mechanisms of injury; (5) Surgery. Surgical reconstruction of unstable injured and traumatized spines. The proposed work has clinical sugnificance in several areas. For the first time, spatial relationships between vertebrae, ligaments, spinal cord, nerve roots and vertebral arteries will be quantified and visually displayed, thereby providing a more thorough understanding of the way in which this joint complex functions. The biomechanical and instability studies will document normal and abnormal motions of the upper cervical spine, which will help determine the instability thresholds for clinical use. Experimental trauma will determined mechanisms of common injuries: dislocations, fractures and whiplash, again helping to provide an understanding of spinal injuries, which may lead to improved treatment methods. Finally, biomechanical evaluations of presently available surgical procedures for stabilizing injured and arthritic spines will lead to recommendations for improvements of present- day methods, as well as to the development of new surgical procedures.